The electronic states of but-2-yne studied by VUV absorption, near-threshold electron energy-loss spectroscopy and ab initio configuration interaction methods

The lowest valence singlet states are ππ∗, optically forbidden, and calculated to lie near 7.3 and 7.6 eV. The states which contribute strongly to the observed spectrum are πσ∗(1E′+1A2″) near 7.9 eV having 2e′6a1′∗ excitation, followed by several ππ∗ and πσ∗ states (1E′+1A2″) between 10.0 and 10.5 eV; an 1E′ antisymmetric combination(2e′2e″ − 2e′2e″) is by far the strongest in intensity. A further group of symmetry-allowed valence states are calculated to lie near 12.3 and 12.9 eV. The two lowest triplet states, both of E′ symmetry (ππ∗), have vertical excitation energies of 5.7 and 6.2 eV, but are strongly bent with a trans-CCCC unit (CS and C2h). The theoretical work confirms that, on intensity grounds, valence excited states do not contribute significantly to the spectrum. CI calculations of the ionic states give the ionisation energy sequence (D3h): 2E′<2A1′<2E″<2E′<2A2″. Adiabatic structures for the first cation, two triplets, and a singlet (C2h) were obtained; these show shortening of C-C, and lengthening of CC, in a trans-CCCC, as is found with ethyne.